This invention relates to a semiconductor test system for testing semiconductor integrated circuits such as ICs and LSIs, and more particularly to a semiconductor test system which can efficiently test semiconductor integrated circuits having PLL (phase lock loop) circuits.
In testing semiconductor devices such as ICs and LSIs by a semiconductor test system, such as an IC tester, a semiconductor IC device to be tested is provided with test pattern data produced by an IC tester at its appropriate pins at predetermined test timings. The IC tester receives output signals from the IC device under test generated in response to the test pattern data. The output signals are sampled by strobe signals with predetermined timings to be compared with expected data to determine whether the IC device functions correctly.
Some IC devices include therein phase lock loop (PLL) circuits for generating internal clock signals. A phase lock loop (PLL) circuit locks the phase of internal clock signal generated by an internal clock oscillator to the phase of a reference clock signal given from an outside source. Examples of IC devices having PLL circuits include microcomputers and RISC processors. One of the advantages of a PLL circuit is that it can produce a clock signal of reduced jitters or phase fluctuations.
FIG. 3 is a schematic diagram showing a structure for testing an IC device having a phase lock loop circuit by a semiconductor test system. In this example, an IC device under test (DUT) 40 includes a phase lock loop (PLL) circuit 45 to produce an internal clock (PLL clock) signal based on a clock signal received at a clock pin 41.
Before going into the relationship between the semiconductor test system and the PLL circuit 45 in the DUT 40, a brief description is made regarding the structure and operation of a PLL circuit. An example of a PLL circuit is shown in FIG. 5 which is comprised of a phase detector (phase comparator) 46, a loop filter 47 and a voltage controlled oscillator (VCO) 48.
The phase detector 46 compares the phase of an input clock signal (CLK) and an oscillation signal (PLL CLK) of the VCO 48 and produces an phase error signal which represents a phase difference between the two signals. Upon receiving the phase error signal from the phase detector 46, the loop filter 47 converts the phase error signal to an averaged DC voltage. Typically, the loop filter 47 is a low pass filter formed by analog or digital components.
The VCO 48 is an oscillator wherein the oscillation frequency is controlled by the averaged DC voltage of the loop filter 47. Because of the negative feedback loop, the PLL circuit controls oscillation frequency of the VCO 48 such that the oscillation frequency of the VCO 48 matches with the input clock signal.
The PLL circuit basically performs a two step operation in which the phase synchronization is reached (lock end) after a pull-in step in which the frequency of the VCO 48 approaches close proximity with the reference clock frequency and a lock-in step in which the phase of the two signals are synchronized with one another. By being applied to an inner clock signal (PLL clock), the PLL circuit can reduce the jitters of the clock signal from external sources, or acts as a clock buffer of zero phase delay.
The configuration and function of the semiconductor test system that tests the DUT 40 having the PLL circuit is explained in the following with reference to FIGS. 3 and 4. The configuration of FIG. 3 shows only functional blocks for generating test pattern data and clock signals to be applied to the DUT 40. The main functional blocks in the semiconductor test system for generating the test pattern data and the clock signal include a timing generator 10, a pattern generator 20, and clock and waveform generators 300 and 301.
The clock and waveform generators 300 and 301 commonly receive various signals from the timing generator 10 and the pattern generator 20. The clock and waveform generator 300 provides a clock signal to the clock pin 41 of the DUT 40 and the clock and waveform generator 301 provides the test data to the data pin 42 of the DUT. Although only one data pin is shown in FIG. 3 for the simplicity of explanation, the DUT 40 usually has many data pins such as several tens to several hundreds pins. Accordingly, in an actual semiconductor test system, a large number of clock and waveform generators 301-30n for data pins are prepared, although not shown here.
The timing generator 10 generates a reference clock RCLK 100, a tester rate signal 200, and a clear signal 300. The reference clock RCLK 100 is a reference clock signal of the semiconductor test system produced by a high stable oscillator such as a crystal oscillator. The reference clock RCLK is used to generate clock edges for producing the tester rate signal 200 and the test pattern data 620. The reference clock RCLK has a frequency of, for example, 100 MHz.
The tester rate signal 200 is also called a test cycle signal and is generated based on desired number of periods of the reference clock RCLK. Generally, timings of test pattern data and strobe signals (not shown) in each test cycle (tester rate) are defined based on a starting edge of the tester rate signal. In a modern semiconductor test system, the time interval of the tester rate signal is dynamically changed under the control of a test program.
The clear signal 300 is to clear (reset) the previous data setting before starting the next set (block) of test patterns. The pattern generator 20 generates the pattern data 600 which includes test data 620 to be applied to the data pin 42 of the DUT 40 and the expected data (not shown) to compare the resultant output of the DUT 40.
The clock and waveform generator 300 generates a clock signal 120 which is applied to the clock pin 41 of the DUT 40. The clock and waveform generator 301 generates the test pattern data 620 which is applied to the data pin 42 of the DUT 40. The clock signal 120 is produced based on the reference clock RCLK and the tester rate signal 200. The test pattern data 620 is produced based on the pattern data 600 with use of the reference clock RCLK 100 and the tester rate signal 200.
The procedure for testing the DUT 40 having the PLL circuit 45 by the semiconductor test system of FIG. 3 is explained with reference to the timing chart of FIG. 4. FIG. 4 is directed to a process for conducting a function test on the DUT. In general, the function test of an IC device under test is carried out by supplying the test pattern data which is divided into a large number of pattern blocks to the IC device. In FIG. 4, before the start of the function test, the PLL circuit 45 in the DUT 40 has to be brought to the lock end (phase lock state) by applying the clock signal 120 to the PLL circuit 45.
In the arrangement of the conventional technology of FIG. 3, the clock signal 120 to the PLL circuit 45 stops when the tester rate signal 200 stops. If the clock signal is not supplied, the phase lock state in the PLL circuit 45 is destroyed (out of phase lock).
As stated above, in general, when a function test is performed for an IC device, the overall test pattern is separated into several hundred blocks to several thousand blocks of test patterns. Thus, the test patterns are generated continuously in the unit of several 10k patterns or several 100k patterns for each pattern block. Prior to the start of each of the blocks of test patterns, the clock signal 120 must be provided to the PLL circuit 45 to bring the PLL circuit 45 to the phase lock state. Since the PLL circuit 45 needs a certain length of time, such as several milliseconds, to reach the phase lock state (lock end) for each block of the test pattern, a significant amount of time is required to phase lock the PLL circuit to complete the function test.
This operational process in the conventional semiconductor test system is shown in the timing chart of FIG. 4. Such an operational procedure is itemized in the following.
(1) Before the start of the test pattern (pattern start) of a test pattern block, the previous data in the clock and waveform generators 300 and 301 is cleared (reset) by the clear signal 300.
(2) The tester rate signal 200 is generated at the start of the test pattern, which produces the clock signal 120 in the clock and waveform generator 300. The clock signal 120 is applied to the clock pin 41 of the DUT 40.
(3) By controlling the pattern PAT 600 supplied to the clock and waveform generator 301, the pattern data 620 applied to the data pin 42 of the DUT 40 is fixed to a LOW level, i.e., the pattern data 620 is not provided to the DUT 40.
(4) When the PLL circuit 45 in the DUT 40 has reached the lock end condition, the test pattern data 620 from the clock and waveform generator 301 is applied to the data pin 42 of the DUT 40. The time required for the PLL circuit to reach the lock state, that is, the time from the pattern start to the lock end, is several milliseconds.
(5) When the tester rate 200 stops at the end of the pattern block (pattern stop), the clock signal 120 from the clock and waveform generator 300 also stops, and the phase lock in the PLL circuit 45 is broken.
(6) The last data in the clock and waveform generators 300 and 301 are cleared again by the clear signal 300. The time required to clear the data, that is, a time length from the pattern stop to the next pattern start, corresponds to a time length for executing each step of the software of the test system, which is several microseconds.
(7) The test patterns are repeatedly generated by repeating the steps of (2)-(6) described above, and the function test for the DUT 40 is performed for all of the pattern blocks of the test pattern.
As in the foregoing, when testing the DUT 40 having the PLL circuit 45 therein, at the start of the test pattern of each pattern block, the PLL circuit 45 must be phase locked. As noted above, there involves as many as several hundred pattern blocks to several thousand blocks in the functional test of the DUT 40, and the time required for reaching the phase lock is relatively long, such as several milliseconds. Thus, the overall test time is increased by the amount of time which is a multiple of the time required to lock the PLL circuit 45 in the DUT 40 and the number of pattern blocks used in the test. Compared to the time required to reach the lock end for the PLL circuit which is several milliseconds, the time required for a clear process is negligible since it is in the order of several microseconds as noted above.
As in the foregoing, when the semiconductor having a PLL circuit is tested by the conventional semiconductor test system, there is a disadvantage that the overall test time is increased by the amount equal to the time required to lock the PLL circuit 45 multiplied by the number of pattern blocks involved in the function test.
Therefore, it is an object of the present invention to provide a semiconductor test system which is capable of testing a semiconductor device having a PLL circuit therein without increasing the overall test time.
It is another object of the present invention to provide a semiconductor test system which is capable of testing a semiconductor device having a PLL circuit with high efficiency by continuously supplying a clock signal to the PLL circuit during the transition period between one pattern block to the next pattern block.
It is a further object of the present invention to provide a semiconductor test system which is capable of continuously supplying the clock signal to a PLL circuit in the semiconductor device throughout all of the test pattern blocks to maintain the phase lock state in the PLL circuit until the end of the overall test.
For testing a semiconductor device (DUT) having a phase lock loop (PLL) circuit therein by supplying pattern data which is divided into a large number of pattern blocks, the semiconductor test system of the present invention is comprised of: a first clock and waveform generator for supplying a clock signal to the PLL circuit in the DUT through a clock pin at a start of the first pattern block, a second clock and waveform generator for supplying pattern data to a data pin of the DUT during each of the pattern blocks, a pattern generator for generating pattern data which is supplied to the second clock and waveform generator based on a test program, and a timing generator for generating a tester rate signal which defines each test cycle in the test system, a clear signal for resetting the data in the first and second clock and waveform generators, and a gate signal for controlling the tester rate signal and the clear signal in the first and second clock and waveform generators, wherein the clock signal is continuously provided to the PLL circuit until the end of the last pattern block while the pattern data to the data pin is reset between the end of the current pattern block and the start of the next pattern block.
In the semiconductor test system of the present invention, the clear signal is inhibited by the gate signal in the first clock and waveform generator during the period between the end of the current pattern block and the start of the next pattern block so that the clock signal is continuously provided to the PLL circuit in the DUT, and the tester rate signal is inhibited by the gate signal in the second clock and waveform generator during the period between the end of the current pattern block and the start of the next pattern block so that the pattern data is not provided to the data pin of the DUT.
According to the present invention, when the function test of the DUT having the PLL circuit is performed, at the start of the first pattern block, it is necessary to establish the phase lock in PLL circuit. Thereafter, the phase lock state in the PLL circuit is maintained until the end of all of the pattern blocks. Thus, during each period between the end of the present pattern block and the start of the next pattern block, only the time to reset the data has to be spent rather than the time required for bringing the PLL circuit to the phase lock state. Hence, the overall test time is dramatically reduced by the amount of time which is a multiple of the time T required to phase lock the PLL circuit 45 in the DUT 40 and the total number N of pattern blocks used in the function test less one pattern block, i.e., T(Nxe2x88x921).